A class of branched, filamentous green algae (division Charophyta), collectively known as the stoneworts, brittleworts, or muskgrasses, which occur mostly in freshwater or brackish-water habitats. Members of the class Charophyceae (often referred to as charophytes) are important as significant components of the aquatic flora in some locales, providing food for waterfowl and protection for fish and other aquatic fauna. They also are excellent model systems for cell biological research, especially as they constitute a unique group of green algae thought to be more closely related to the land plants. See also: Algae; Chlorophycota; Plant kingdom; Thallobionta

Variously shaped nodules consisting of fossilized peat in which the individual cells and tissue systems of the plant parts are infiltrated by minerals, principally calcium carbonate, along with pyrite, dolomite, and occasionally silica. This type of fossilization, in which the cell walls are filled with minerals, is termed permineralization. Coal balls occur principally in Pennsylvanian (upper Carboniferous) bituminous and anthracite coals, often below black marine shales in North America, but permineralized plants have also been reported in coals from as early as the Devonian and extending well into the Paleocene.

A group of unicellular, biflagellate, golden-brown algae characterized by a covering of extremely small (1–35 micrometers) interlocking calcite (the hexagonal form of calcium carbonate) plates called coccoliths. The plates show extinction crosses in a polarizing light microscope; however, detailed study requires the use of electron transmission or electron scanning microscopes. The Coccolithophorida are usually considered plants but possess also some animal characteristics. Botanists assign them to the class Haptophyceae (based on the possession of a haptonema, a threadlike organ of attachment) of the phylum Chrysophyta and zoologists to the class Phytamastigophorea, superclass Mastigophora, subphylum Sarcomastigophora of the phylum Protozoa. See also: Cilia and flagella; Phytamastigophorea; Protozoa

True ferns which span the Late Devonian through Permian time between the recognizable beginnings of fernlike morphology and the earliest-appearing extant filicalean families (Gleicheniaceae, Osmundaceae). A true fern is a relatively advanced type of vascular land plant with distinct stem, fronds, roots and foliarborne annulate sporangia. Coenopterid ferns are mostly small and simple in contrast to late Paleozoic tree ferns of the Marattiales. All well-known genera of the Coenopteridales in the Pennsylvanian Period exhibit fronds with circinate vernation (croziers), laminate vegetative foliage and annulate sporangia. Their stems are mostly protostelic with a few siphonostelic species and one genus (Zygopteris) has secondary growth. These ferns are better from anatomically preserved specimens, especially from peat beds, than from coalified compressions, but they occurred in a variety of habitats mostly known on Northern Hemisphere continents. Their fronds have distinct vascular strands, useful in identifying genera (Fig. 1).

Any living or fossil member of the plant division Pinophyta, consisting chiefly of woody trees or shrubs that produce cones and evergreen needles. Conifers are predominantly cone-bearing, evergreen trees and shrubs belonging to the order Pinales, as well as a small number of extinct orders, in the class Pinopsida of the plant division Pinophyta (Coniferophyta). All conifers are woody plants, and they are often the predominant trees of forests worldwide (Fig. 1), including pine, spruce, fir, hemlock, cedar, larch, juniper, cypress, yew, and redwood species. Because the ovules (young seeds) of conifers are exposed directly to the air at the time of pollination (that is, conifers produce seeds that are not enclosed in a fruit or ovary), these plants are classified as gymnosperms. (In contrast, the flowering plants, or angiosperms, have ovules enclosed in an ovary.) Today, conifers constitute the principal source of lumber and pulp for paper and wood products. See also: Cedar; Cypress; Fir; Forest; Forest timber resources; Forestry; Hemlock; Juniper; Larch; Lumber; Pinales; Pine; Pinophyta; Pinopsida; Redwood; Seed; Spruce; Tree; Wood products; Yew

An order of red algae, commonly called coralline algae. The order Corallinales (class Rhodophyceae; phylum or division Rhodophyta) consists of three families: Corallinaceae, Hapalidiaceae and Sporolithaceae. Coralline algae (see illustration) are distinguished by the impregnation of cell walls with calcite, a form of calcium carbonate, which causes the thallus to be stony or brittle. Other types of red algae that are calcified are much softer than corallines and are impregnated with aragonite, another form of calcium carbonate. Many early biologists (including Carolus Linnaeus) thought that these plants were related to corals. See also: Algae; Aragonite; Calcite; Calcium; Carbonate minerals; Rhodophyceae

An extinct order of the class Pinopsida comprising a natural grouping of Paleozoic forest trees or shrubs that first appeared in the Lower Pennsylvanian. They became an important component of the tropical vegetation during the Middle and Upper Pennsylvanian and diminished during the basal Permian. The Cordaitales were divided into three families, Cordaitaceae, Pityaceae and Poroxylaceae. However, members of the Pityaceae and Poroxylaceae are now known to be either seed ferns or progymnosperms, so only the Cordaitaceae remain. Detailed information about these prominent Paleozoic gymnosperms comes from impression-compression fossils and sandstone casts. However, coal balls, which are mineral nodules in which the plants are preserved with three-dimensional anatomy, have been essential to establishing whole-plant concepts of cordaitean species. See also: Coal balls

An order of gymnosperms in the class Ginkgoopsida with only one extant species, Ginkgo biloba (the maidenhair tree). Ginkgo biloba (Fig. 1), also referred to as the ginkgo tree, is native to China. It is cultivated as a shade tree in cities around the world because of its beauty and its resistance to disease and the effects of automotive pollution. It is the only surviving species of the class Ginkgoatae and is considered a living fossil. For thousands of years, the leaves and seeds of the ginkgo tree have been used to make herbal extracts in traditional Chinese medicine. More recently, these extracts have been introduced into Western medicine. See also: Ginkgoopsida; Living fossils; Tree

A class of largely extinct gymnosperms. The class Ginkgoopsida contains one order of seed plants: Ginkgoales. Representatives of the Ginkgoales lived predominantly from the Triassic into the Cretaceous, with one species, Ginkgo biloba, persisting to the present. See also: Cretaceous; Ginkgoales; Triassic

The study of fossil plants of the geologic past. A paleobotanist is a plant historian who carefully pieces together the geologic history of the plant kingdom. Other organisms, including fungi and various types of microscopic plankton, are also studied by paleobotanists. Paleobotany is a branch of paleontology that requires a knowledge of both plant biology (botany) and the geological sciences. See also: Botany; Fossil; Plant kingdom